Difficult problems are all made up of simple questions. First, you need to fully understand the examples in the book and the exercises after class; You are not fit to do comprehensive exercises now. It seems that your foundation is not very good You can find your own weaknesses and do this part of the exercise you bought before. If you really want to buy a book, what you did then was a three-year and five-year college entrance examination simulation, and 38 sets of Tian Li.